This invention relates to optical communication systems in which electro-optical emitters and detectors are coupled to the ends of optical fibers for sending and receiving signals along the fibers. More specifically, this invention relates to those communication systems in which individual fibers carry signals bidirectionally.
Because optical fibers can transmit information at much greater rates than copper wire, there is much interest in schemes for delivering telecommunication services to customer premises, such as residential homes, over optical fiber. These schemes fall into two general classes, depending on whether each terminal location receives transmissions from the central office over its own dedicated line, or whether multiplexing is used to reduce the number of fibers that fan out from the central office. The first scheme is said to have Point-to-Point (PTP) Architecture. When passive optical components are used at intermediate locations to demultiplex downstream signals (and, in some cases, to multiplex upstream signals), the second scheme is said to have Passive Optical Network (PON) architecture.
One problem encountered by designers of PTP networks is fiber congestion at the central office or active remote node (ARN), where downstream signals are placed on optical fibers. There is a need for line cards with high-density electro-optical interfaces, to alleviate this congestion.
One problem encountered by designers of both PON and PTP networks is the need for expensive equipment at the fiber terminations. That is, bidirectional coupling of signals into and out of the end of an optical fiber typically calls for optical splitters and couplers that are bulky and expensive to manufacture. In PON networks in particular, this is a problem for fiber installations within the customer premises. Typically, a fiber extends from the on-premises optical network unit (ONU) to a network PON fiber termination defining the physical interface between the network and the customer premises. There is a strong economic incentive to reduce the cost of the interfaces at the ends of this fiber.
We have discovered that these, and other, problems of bidirectional communication in optical networks can be alleviated by using dual concentric core fiber (DCCF) to carry optical signals bidirectionally. In one broad aspect, our invention involves a method for communicating messages, in the form of modulated optical signals, over an optical fiber. Inbound messages lying in a first wavelength channel are received from a terminal portion of the fiber, and outbound messages lying in a second such channel are injected into the terminal portion. The optical fiber has at least one annular portion surrounding a central core portion. The inbound messages are received from the annular portion, and the outbound messages are injected into the central core portion. In alternate embodiments of the invention, the inbound messages are received from the central core portion, and the outbound messages are injected into the annular portion.
As used herein, the directions indicated by the terms xe2x80x9cinboundxe2x80x9d and xe2x80x9coutboundxe2x80x9d are relative to the terminal device under discussion, irrespective of whether the signals so described are directed toward or away from the central office. Those directions are indicated, respectively, by the terms xe2x80x9cupstreamxe2x80x9d and xe2x80x9cdownstreamxe2x80x9d.